The integrated circuit (IC) has introduced significant changes and improvement in electronic equipment design and manufacture. Hybrid integrated microcircuits are generally of two type -- thin film or thick film. Thin film IC's comprise a plurality of components such as transistors, resistors, and capacitors incorporated in planar form on a silicon or high quality ceramic substrate. Some circuit elements are grown or formed on the substrate while others are deposited in an evacuated atmosphere, generally by sputtering or evaporation. The major disadvantage of thin film circuits is the high cost of the vacuum processing technology required. High valued capacitors are not available in this technology since large dielectric constant dielectrics are not practical.
Thick film hybrid circuits are generally characterized by a relatively inexpensive ceramic substrate and passive elements (resistors, conductors and capacitors) which are screen printed, in layers, and fired at high temperature as opposed to being vacuum deposited. One practical advantage is that thick film resistors are available in a wide range of values and they may be more readily trimmed to value than their thin film counterparts. Thick film circuit capacitors, may also be made with relatively high dielectric constants, up to 1000. However, the dielectric is porous and the capacitor should be hermetically sealed to prevent moisture from permeating the dielectric. Moisture in the dielectric can cause metal migration and shorting of electrodes when the capacitor is under an electrical field.
Ceramic type capacitors, which have been known for some time and have proven very satisfactory, often include a barium titanate-based dielectric. The normal 1350.degree. C firing temperature for barium titanate dielectric materials introduces problems in thick film circuit manufacture. For commerical use, such as in television receivers, it is imperative that production costs of hybrid circuits be minimized and automated equipment is used wherever feasible. In modern thick film manufacturing equipment, the screened substrates are automatically fed through a belt-type furnace and stacked at the end of processing. However, the wire mesh belts used in the production equipment oxidize very rapidly at temperatures above 1050.degree. C and experience rapid deterioration, leading to very short life.
In "pusher" type furnaces, ceramic slabs (which are not adversely affected by the high temperatures) are forced through the heating chamber. Not only are these furnaces expensive, they are very difficult to automate and to control. Their normal range of temperature is .+-.10.degree. C, whereas for thick film processing, the temperature should be controlled to within .+-.2.degree. C. The requisite temperature control may be attained by using a "batch" process, at the expense of the automation feature necessary to obtain reasonable production economy.
Barium titanate particles, when fired at 1350.degree. C, fuse and become a strong dense material which, when used as a dielectric for a discrete capacitor, need not be sealed to prevent moisture penetration.
When fired at low temperatures, very little fusing of particles occurs and the material is extremely porous and fragile. As such it must be sealed. Mixing with a low temperature fusing glass improves both its porosity and strength charcteristics, but not sufficiently to avoid the need for sealing.
In addition to the reasons enumerated above, firing at lower temperatures permits use of other less costly materials. For example, in an air atmosphere, high firing temperatures dictate that the conductive material have a greater concentration of precious metal than when low firing temperatures are used. The conductive material used for the capacitor electrodes and conductors, when fired at 1000.degree. C, is a silver palladium alloy. At a firing temperature of 1350.degree. C, the conductive material is either palladium gold or platinum palladium. Either one of these two latter conductors is much more expensive than silver palladium conductors.
Another problem in using barium titanate as a dielectric material is the large disparity between the temperature coefficients of expansion (TCE) of the alumina substrate and the dielectric. Even at lower firing temperatures and using a mixture of glass and barium titanate particles, some cracking of the dielectric layer is still experienced. Minor cracking of the dielectric is not harmful if a suitable seal is provided.
The sealing problem for thick film capacitors is much different from that for normal "discrete" ceramic capacitors. In the latter each unit is inherently dense and atmospheric moisture may not penetrate it. A simple organic coating prevents surface conduction. In a thick film circuit the capacitors have porous dielectrics and they must be sealed against atmospheric moisture.
Thick film capacitors readily fail when subjected to an electrical field under high humidity conditions. The failure apparently occurs because of the formation of metallic ions in the presence of water vapor which subsequently migrate from the negative electrode to the positive electrode. The ions thereupon convert to the metallic form and a dendritic buildup of the metal occurs on the positive electrode until a short develops through the dielectric. It has been found in the case of an AgPd conductor with a Bi.sub.2 O.sub.3 glass frit system that the migrating metal is bismuth. Apparently the bismuth is ionized and transported to the positive electrode by the DC field gradient. It is de-ionized there and bismuth metal dendrites form and grow toward the negative electrode. When the bismuth reaches the negative electrode, a short develops. The failure thus seems to require a source of ions, a porous dielectric and an electrical field. If moisture is prevented from reaching the dielectric, the metallic ions will not form and this type of failure should not occur.
Organic film coatings are too permeable to moisture to protect the capacitor. Bulky organic encapsulants are not practical since they are very expensive and also may interfere with leads and components to be attached to the substrate. It has been found that thick film capacitors may be protected with glass films which have very low permeability rates to water vapor as well as to all gases.
Attempts to seal the thick film capacitor with a liquid glass failed because the glass actually flows into the pore structure of the top electrode and dielectric layer and lowers the capacitance. Viscous glass sealants also result in poor seals and subsequent degradation of the capacitor under atmospheric conditions, believed mainly due to voids and bubbles in the glass and inadequate glass-to-substrate bonding. The TCE of the sealing glass also poses a problem in that if the disparity between it and the TCE of the underlying support layer substrate is great, cracking will occur. This, of course, results in an unusable device because of seal failure. Still other attempts were directed at producing a barrier between the capacitor, per se, and the sealing glass to avoid glass penetration of the dielectric. While these attempts met with varied degrees of success, the "yields" were unsatisfactory for production purposes.
As mentioned, generally each capacitor is individually sealed, although they may be grouped so as to have several with a common seal system. When it is considered that a thick film integrated circuit may need as many as 30 capacitor seals, the yield for each capacitor seal must be extremely high to get a reasonably acceptable yield of finished substrates. Since the processed substrate is subjected to active component attachment and soldering (as well as resistor trimming) it cannot have an overall seal coating. Thus the yields of each capacitor seal must be successively multiplied by all capacitor seals to obtain the substrate yield for capacitor seals.